1. Field of the invention
The present invention relates to a lighting device using an argon lamp as the light source, in particular, to a lighting device wherein an argon lamp is lighted by energizing a voltage exceeding its rating.
2. Description of the prior art
Recently, the number of school children with pseudomyopia and workers with eyestrain is steadily increasing. This would be due to long-time TV and fine print watching under inappropriate lighting, as well as to increasing VDT task. Recent tendency prefers lighting devices which makes a person to recognize the color of an article as it really is, and repeated discussions have been focused on color rendering properties of lighting.
Light sources which have been extensively used in lighting devices are fluorescent and incandescent lamps. Fluorescent lamp has the merit that its relatively high total flux provides a bright lighting, as well as having the demerits that its flicker tends to cause eyestrain, and that the unbalance between total flux and color temperature deteriorates the color rendering properties to make articles look paler than they really are.
Although incandescent lamp is superior to fluorescent lamp because the well balance between total flux and color temperature achieves satisfiable color rendering properties, incandescent lamp has the demerit that it is generally low in total flux and this tends to cause eyestrain when used in lighting for long time.
In order to overcome these drawbacks of conventional lighting device, the present inventor investigated incandescent lamps having superior color rendering properties. As the result, the present inventor found that a light obtained by applying to an incandescent lamp a voltage exceedings its rating attains a color temperature approximately the same as that of early morning sunlight (about 3,000 K.) which is comfortable to the eyes, as well as that such a light is suitable for general lighting. The present inventor disclosed these in Japanese Patent Laid-Opens No.193,398/86, No.185,516/87, No.26,909/88 and No.88,792/88.
The present inventor also found that in the case of argon lamps wherein argon gas is enclosed as the filling gas, many of the argon lamps as tested, however, emitted no natural light which had both a color temperature of about 2,900 K. or higher, desirably, in the range of about 2,950-3,050 K., and superior color rendering properties when energized with an overvoltage which fell within a practical range chosen in consideration with service life and safeness.